Double vision, or diplopia, is a condition where a person sees two distinct images of a single object. This occurs because the eyes are not working together seamlessly. Prism glasses are a common, non-invasive optical solution often prescribed to manage this symptom by correcting the path of light entering the eyes. The specialized lenses do not fix the underlying cause of the misalignment but provide immediate relief by tricking the brain into perceiving a single, clear picture. This article explores the mechanism by which prism glasses achieve this correction and how they fit into the broader treatment landscape for double vision.
What Causes Double Vision
Binocular double vision arises when the eyes fail to point at the same target simultaneously, a condition known as strabismus or heterophoria. For clear, single vision, light must fall on corresponding points on the retina of each eye, allowing the brain to fuse the two inputs into one coherent image. When the eyes are misaligned, the image lands on a different point in each eye, leading to the perception of two images that may appear horizontally, vertically, or obliquely separated.
The root causes of this eye misalignment involve issues with the six extraocular muscles that control eye movement. Conditions like Graves’ disease, myasthenia gravis, or cranial nerve palsies can weaken these muscles or impair the nerves controlling them, preventing the eyes from tracking together. Neurological events such as a stroke, head trauma, or multiple sclerosis can also disrupt the brain’s control over eye coordination. Because double vision can be a sign of a serious underlying health condition, a thorough medical evaluation is necessary to determine the exact etiology.
How Prism Glasses Redirect Light
Prism glasses rely on refraction, the bending of light as it passes through a medium. A prism is a wedge-shaped piece of glass or plastic with a thick edge, called the base, and a thin edge, called the apex. When light rays pass through this lens, they are always bent toward the thicker base.
The prism is precisely oriented in the glasses to compensate for the eye’s misalignment. If an eye is deviating outward, the prism is placed with its base facing outward to shift the incoming image back toward the center of the retina. By redirecting the image so it lands where the brain expects it, the visual system can successfully fuse the two separate signals into a single, comfortable image.
Prescribing and Different Types of Prisms
The amount of correction required is measured in a unit called the prism diopter. One prism diopter is defined as the power needed to shift an image one centimeter for every one meter of distance. The full prescription specifies both the power in prism diopters and the direction of the prism’s base, which is crucial for proper placement. The base direction depends on the eye misalignment and is specified as:
- Up (BU)
- Down (BD)
- In (BI, toward the nose)
- Out (BO, away from the nose)
Ground-In Prisms
A ground-in prism is permanently manufactured into the lens material itself, resulting in a clearer, more cosmetically appealing lens. These are preferred for long-term correction when the required prism strength is stable.
Fresnel Prisms
In contrast, a Fresnel prism is a thin, flexible plastic sheet with concentric rings that is pressed onto the surface of a regular eyeglass lens. Fresnel prisms are often used for temporary correction or diagnostic testing because they are inexpensive and can be easily applied or removed. They are particularly useful when the double vision is fluctuating or when a very high amount of prism is needed, which would make a ground-in lens extremely thick and heavy. However, Fresnel prisms can reduce visual clarity and may be noticeable due to the visible lines on the surface, making them less ideal for a permanent solution.
Prisms as Compensation Versus Cure
Prism glasses offer a compensatory solution rather than a cure for the underlying cause of the eye misalignment. The lenses manage the symptom of double vision by altering the light’s path, but they do not resolve the muscle or nerve issue causing the eyes to deviate. This distinction means that if the underlying condition progresses, the required prism strength may change over time, necessitating new lenses.
For patients with unstable or fluctuating conditions, such as those recovering from a stroke or managing certain neurological disorders, the prescription may need frequent adjustments. In cases where the misalignment is too significant or unstable for prisms alone, other interventions may be necessary. Alternatives include vision therapy, which uses targeted eye exercises to improve eye coordination, or surgical intervention to physically adjust the extraocular muscles. For many people, a combination of prism correction and vision therapy provides the most comprehensive approach to achieving comfortable, single vision.